Metal plated films have been widely employed for wiring patterns provided on a mounting board as well as pads at the sites for jointing between semiconductor devices and wiring patterns, which are used in assembling of semiconductor devices so far. Along with recent improvement in the technique of drawing fine patterns using inkjet printing, application of formation of wiring pattern using a conductive metal paste is under way. In addition, application to finer wiring patterns is now advanced by making the particle size of the metal filler used in the conductive metal paste much smaller.
On the other hand, there are established methods of producing metal ultrafine particles having an extremely small particle size, such as metal ultrafine particles with at least their average particle size of 100 nm or less. For example, JP 3-34211 A1 discloses a dispersion in which ultrafine particles of 10 nm or less prepared by a gas evaporation method are dispersed in a dispersion solvent in the form of colloid and a process for producing the same (see JP 3-34211 A1). Further, there have been disclosed, for example in JP 11-319538 A1, colloid dispersions of ultrafine particles having an average particle size of several nm to several 10 nm prepared by a wet process utilizing a method of reduction/precipitation with an amine compound for reduction and a process for production thereof (see JP 11-319538 A1).
As such metal nanoparticles having a nanosize average particle size have been employed as a conductive medium, a conductive layer formed by using such conductive paste is now applicable to formation of extremely fine wiring pattern. When a metal filler of several microns in the size is used for conductive metal paste, the metal filler particles are closely adhered and fixed with each other by using a binder resin to create electrically conductive paths. On the other hand, when a similar technique for making physical contact of particles is used for ultrafine metal particles having an average particle size of several nm to several 10 nm, the smaller the average particle size, the more remarkable the increase in the entire resistivity due to contact resistance.
Generally, ultrafine metal particles having an average particle size of several nm to several 10 nm are known to be sintered at a temperature well below the melting point (for example, even at 200° C. or lower in the case of silver ultrafine particles having a clean surface). This is because, when the particle size of ultrafine metal particles is sufficiently small, the proportion of atoms in a high energy state present on the particle surface increases in the whole particle, and thereby the surface diffusion of metal atoms is increased to a far from negligible level, and as a result, the boundary between the particles are grown wider due to this surface diffusion to cause sintering. In the case of using metal nanoparticles as a conductive medium, this characteristic of being sinterable at low temperatures is utilized to form a network-like sintered product layer of metal nanoparticles closely contacted with each other by sintering, and as a result, the increase in the entire resistivity due to contact resistance is suppressed and such a good electrical conductivity is achieved as a lowered specific resistivity of about 10×10−6 Ω·cm for the entire sintered product layer of metal nanoparticles.
It is desired that when used wiring patterns are further miniaturized with narrower line widths, the thickness of the wiring layer in the wiring part with the narrower line width is thicker relative to the line width to increase the thickness/width ratio in the cross-section of the wiring layer in order to prevent the rapid increase in the resistance of the wiring. For example, in the case of metal plated films formed by means of electrolytic plating, metal is deposited even on the edges of the line when the line width is narrower and the plating thickness is increased; and therefore it is difficult to form a plated film having a narrow line width and a high thickness/width ratio with maintaining integrity of the desired finer wiring pattern. On the other hand, in the case of wiring pattern formation using a conductive metal paste, when the fluidity of the conductive metal paste used is high, the paste runs off from the edges on both sides of a line due to the high fluidity of the paste even if the paste is coated at the intended thickness/width ratio. Thus, it is also difficult to form a conductive metal paste coating layer having a narrow line width and a high thickness/width ratio with maintaining integrity of the desired finer wiring pattern.
As a technique for forming a thick plated film with an edge being kept in sharp shape, a method is known in which a mask for plating is beforehand prepared using a resist film or the like and then an electroplated film is formed in alignment with the opening shape of the plating mask. In that case, it is necessary that such a mask for plating having an opening with a high thickness (depth)/width ratio for the narrower widths should be provided, but there has been not yet developed a technique having wide applicability that can form easily such opening with a high thickness (depth)/width ratio by using a resist film. Likewise, in the case of wiring pattern formation using a conductive metal paste, leak-out from the edge on both sides of a line can be prevented when a mask formed by using a resist film is used, but there has been not yet developed a method of preparing mask usable for various purposes that can form easily an opening with a high thickness (depth)/width ratio. In particular, when the proportion of the dispersion solvent contained in the used conductive metal paste is increased to obtain high fluidity, vapor of the dispersion solvent forms accidentally bubbles inside the opening groove at the stage of removing the dispersion solvent contained in the coating layer by evaporation after application, which may be a main cause resulting in incident of voids.
At least, there has not been yet developed a technique of easily forming a conductive layer having a cross-section of a high thickness (depth)/width ratio, e.g., a pillar-shape having a circular base in which the height is equal to or greater than the radius of the base, by using a conductive metal paste dispersion with a high fluidity that is suitable for application by deposition or inkjet, by means of a method of forming a wiring pattern using a conventional conductive metal paste.